(1) Incorporation by Reference of Related Application
The following related copending application, Ser. No. 07/796,461, filed on Nov. 22, 1991, entitled Multi-frequency Parallel Read-Write Optical Tape Recording System and assigned to the assignee of the present invention is incorporated by reference for its teachings of the environment in which the present invention has advantageous use. The above referenced application, Ser. No. 07/796,461, is a continuation-in-part of U.S. patent application Ser. No. 07/548,106, now abandoned, filed Jul. 5, 1990 titled Linear Digital Tape Recorder, which is assigned to the assignee of the present invention and which is a continuation-in-part of U.S. patent application Ser. No. 07/480,646, now abandoned filed Feb. 15, 1990, titled Multiple Source Optical Tape Recorder, which is assigned to the assignee of the present invention and which is a continuation-in-part of co-pending U.S. patent application Ser. No. 07/405,948, now U.S. Pat. No. 5,120,136, filed Sep. 12, 1989, titled Optical Tape Recorder, also assigned to the assignee of the present invention.
The present invention further relates to U.S. patent application Ser. No. 07/613,396 filed Nov. 15, 1990 titled Chirp Compensation for Acousto-optic Deflection System, which application is assigned to the assignee of the present invention.
(2) Field of the Invention
This invention relates to a phase and amplitude control circuit for signal frequencies used as input to an acousto-optic deflector. More particularly, this invention relates to systems using acousto-optic deflectors for deflection of beams of light in various applications such as optical data storage on tape or disk, optical printing and imaging, and acousto-optic signal or information processing.
(3) Prior Art
Acousto-optic deflectors (AODs) can be used for deflecting light sources in various applications such as optical data storage on tape or disk, optical printing and imaging, and acousto-optic signal or information processing. In such devices an acoustic wave is generated from an electrical signal by an electro-mechanical transducer attached to one end of an acoustic medium which has an acoustic termination at the other end to suppress reflected acoustic waves. A source of light is collimated and directed such that it passes through a region of the acoustic medium which contains the acoustic waves resulting in a portion of this light beam being deflected by interference with the acoustic waves.
It is well known that a collimated optical beam properly directed through an AOD will be deflected through an angle proportional to the frequency of the electrical signal input to the AOD. (A reference on this subject generally is entitled, "Acousto-Optic Signal Processing", by Norman J. Berg, and John N. Lee, Marcel Dekker, Inc., New York, TA1770.A225, 1983, ISBN 0-8247-1667-1.) Furthermore, an input signal consisting of a sum of multiple frequencies will result in multiple deflected beams being output from the AOD with each separate beam corresponding to one of the original input frequencies that were summed. Additionally, the intensity of each separately deflected beam is proportional to the amplitude of the frequency component that produced that beam.
In order to obtain maximum optical efficiency so that the deflected light beams are at their maximum intensity, it is necessary to provide a signal to the electrical input of the AOD that is near the maximum power level at which the AOD will operate without damage. However the relationship between electrical input power and optical power of a deflected light beam is not linear when an AOD is operated at high relative power levels. When a signal is composed of multiple separate frequencies this nonlinear relationship will result in interference between one frequency and another such that the optical power of each separately deflected beam will vary as a function of the sum and difference frequencies of each separate frequency and all other frequencies in the sum signal.
This intermodulation distortion and the resulting variation in optical power of an individual beam will cause serious problems in applications where the optical power of an individual beam is required to remain constant or predictable such as optical data storage on tape or disk, optical printing and imaging, and acousto-optic signal or information processing.
One way to control the intermodulation distortion problem described above is to limit all input frequencies to exact integer multiplies of a common reference frequency, thereby limiting the sum and difference frequencies to just one frequency which is equal to the common reference frequency. This will make the intensity variation of the deflected light beams more predictable but will not reduce or eliminate such intensity variations.
What is needed in order to adequately mitigate this intermodulation problem is a means of limiting maximum signal power while at the same time maximizing average power input to an AOD. The present invention provides a means to achieve this solution by relative phase control of each frequency of the multi-frequency input signal.
In accordance with the present invention a scheme is described which allows a maximum level of electrical power to be input to an acousto-optic deflector (AOD) while at the same time minimizing the maximum power (maximum envelope power) that is input to the AOD, resulting in minimal interference between separately deflected light beams from the AOD. Such minimal interference between separately deflected light beams and their associated input signal frequencies is achieved by precise control of the frequency and relative phasing of each separate frequency relative to a common reference frequency. In accordance with the present invention, the relative phase of each separate frequency is also controlled so that a low maximum power is achieved for the combined signal that is presented to the AOD while maintaining overall average power.
The present invention may be a advantageously utilized in systems for storage and retrieval of information. Numerous prior art systems are available for the storage and retrieval of information. In each of these systems, certain objectives are clear. It is important to provide for storage and retrieval of information at high speed while maintaining the cost of the storage media and associated hardware at a low cost.
Presently, demand for data storage and retrieval capability is increasing and this increase in demand is expected to escalate in the future. There are numerous reasons for the expected increase in demand for data storage capacity, such as the need to store bit-mapped images of documents, etc. Therefore, it is desired, as one aspect of the present invention to provide more control and accuracy within storage systems which provides for more efficiency, higher speed at lower cost.
One emerging technology useful for the storage and retrieval of information is optical storage systems. Such systems allow for storage and retrieval of information on a medium through use of a light source. A well-known example of an optical storage system is an optical disk. Optical disks are utilized in a variety of functions such as recording of digitally encoded music, permanent storage of data for computer systems, etc.
Flexible optical tape, as opposed to optical disk, is commercially available in the market. Optical tape allows for storage of information on optical media with virtually limitless capacity. For Example, non-erasable write once read many (WORM) optical tape is currently available from ICI Imagedata, P.O. Box 6, Shire Park Bessemer Road, Welwyn Garden City, Herts AL7 1HD, ENGLAND. The particular tape manufactured by ICI is available in 35 mm width allowing data to be laser written in a center section at 30 megabytes per square inch (roughly the equivalent in a square inch of medium to 100 standard 51/4 inch double-sided double density floppy disks). The present invention may work with either erasable or WORM recording media. It is also understood that optical tape is commercially available from DOW Chemical Company and in addition, magneto-optic (phase change) media is available from the 3M Company and erasable dye polymer media is available from Kodak. Finally, optical storage optimally suited for use with large linear arrays is available under the tradename ETOM.TM. (Electron Trapping Optical Memory) from Quantex Corporation. Such media may be described as an erasable optical storage media based on stimulated electron transitions. Each of these media may be utilized as a recording media by the present invention.